In order to accurately lock long intramedullary nails (i.e. those with distal attachment screws) the distal screws have to be accurately aligned with the cross-bores in the nail. This locking is complicated by the deflection of the nail during insertion into the bone canal which changes the location of the cross-bore from their static position. Typically the surgeon has been forced to do this freehand with the help of an x-ray C-arm. A common problem in such a procedure is that the instruments are “in the way” since they are on the image plane of the C-arm. Furthermore, distal locking is problematic since the distal bores cannot be made precisely through the soft tissue due to the anatomical shape of the femur and the resulting curvature of the nail (here in a Z direction) which is in a plane perpendicular to a plane parallel to the frontal plane.
Intramedullary nails often provide two distal openings or cross-bores for distal locking. For distal locking a nail may offer three locking options to be used, depending on the fracture pattern. To accomplish this a proximal round hole is provided and a more distal oblong hole. Distal locking is recommended if the fracture is unstable, if rotational stability is required or if there is a wide disparity between the diameter of the nail and the femoral cavity.
The first possibility is placing a locking screw in the distal part of the oblong hole. This creates a dynamic locking mechanism i.e. allows the nail to move distally and requires only one screw. Alternatively, one screw may be placed in the distal part of the oblong hole and the other in the round hole. This causes a static locking of the nail and prevents movement of the nail. However, if dynamization is required after a period of time, the screw, placed in the round hole, may be removed leaving only the screw at the distal end of the oblong bore. This method requires two screws. Lastly, one screw may be placed in the round hold and the other placed in the proximal part of the oblong hole. Again this produces static locking and requires the placement of two screws.
Various techniques can be used to guide drilling and insertion of screws through the distal holes. The freehand technique described above as well as targeting instruments such as used in a straight on approach of the imaging device described below.
The essential initial step in distal targeting is to position the fluoroscope so that the circular distal hole in the nail appears perfectly round. Naturally, this visualization cannot be used with the oblong hole. If the round hole appears to be elliptical in either the vertical or horizontal plane, the fluoroscope image position must be adjusted appropriately. It is advised to correct the image in one plane at a time.
Once an image intensifier is correctly positioned a tip of a drill is placed at the center of the hole and a hole drilled through the first cortex which in a femur is the lateral cortex and the nail cross-bore until resistance of the second cortex is felt. The drill typically has a scale for measuring the required screw length.
Alternatively, a hole can be drilled through the second cortex while viewing the image. The required screw length can then be read directly from the screw scale on the drill. If a tissue protection sleeve is used around the drill, it has to be removed for the measurement. It is also possible to measure the correct screw length using a free hand screw gauge which can engage the medial cortex outer surface when the nail is in the femur. This is done after drilling through the second cortex by removing the drill and advancing the small hook of the screw gauge through the holes behind the medial cortex and read out the required locking screw length.
Typically the distal locking screw, which is usually a 5 mm screw, is inserted through the skin by using a screwdriver. The screw head is advanced carefully until it is just in direct contact with the cortex. Any targeting instrumentation used is then removed.